1. Field of the Invention
The invention relates to an outer lead bonding apparatus and a method for bonding a plurality of leads extending outwardly of a semiconductor chip to bonding pads mounted on a substrate. The invention further relates to a bonding-aid member to be used in the above mentioned apparatus and method.
2. Description of the Related Art
When a semiconductor chip is to be bonded to a circuit substrate, a plurality of leads placed on a carrier tape are first bonded to a plurality of electrodes formed on a surface of a semiconductor chip. Such a bonding is called inner lead bonding (ILB). Then, a plurality of leads extending outwardly of a semiconductor chip are cut so that all of the leads have a predetermined length. Then, distal ends of the leads are bonded to a plurality of bonding pads mounted on a wiring layer of a circuit substrate. Such a bonding is called outer lead bonding (OLB).
FIG. 1 illustrates an example of conventional outer lead bonding. A semiconductor chip 10 having leads 12 adhered to a lower surface of the semiconductor chip 10 and extending outwardly of the semiconductor chip 10 is vacuum-attracted to a jig 83 housed in a bonding tool 82. Then, a position detecting camera 63B detects the position of bonding portions 13 of the leads 12 at which the leads 12 are to be bonded to later mentioned bonding pads 32 mounted on a circuit substrate 30.
The circuit substrate 30 on which the semiconductor chip 10 is to be mounted is fixedly placed on a stage 81 which is movable in two directions perpendicular to each other and rotatable about a central axis thereof. The position of bonding pads 32 mounted on the substrate 30 is detected by a position detecting camera 63A at a location away from the semiconductor chip 10.
Then, in order to align the bonding portions 13 of the leads 12 to the bonding pads 32 mounted on the circuit substrate 30, the positions of are detected independently of each other, and one of them is moved towards another. For instance, the stage 81 is moved in a direction indicated by an arrow 85 to locate the circuit substrate 30 below the semiconductor chip 10. Namely, the stage 81 and hence the circuit substrate 30 is moved in a position indicated with a broken line in FIG. 1. Then, the bonding tool 82, having been heated at about 400 degrees centigrade is lowered to thereby exert about 200 grams load on every lead 12. Thus, the bonding portions 13 of the leads 12 are connected to the bonding pads 32 mounted on the substrate 30.
However, this method has a defect, in that since the jig 83 is housed in the bonding tool 82, the bonding portions 13 of the leads 12 and the bonding pads 32 of the substrate 30 are hidden by the bonding tool 82 while the bonding operation is being carried out. Thus, it is impossible to look at the bonding portions 13 of the leads 12 and the bonding pads 32 at the same time, and hence they have to be detected with respect to a position thereof independently of each other, and then one of them has to be moved to another.
As a result, it is not possible to directly confirm the alignment of the bonding portions 13 and the bonding pads 32 while the bonding is being carried out. Accordingly, even if misalignment occurs, the bonding is carried out anyway with the result being defective bonding.
An attempt for overcoming such a problem has been made in Japanese Unexamined Patent Public Disclosure No. 4-277641. FIG. 2 illustrates the method suggested in the Disclosure No. 4-277641. In that method, there is provided a jig for fixing a semiconductor chip in place, the jig comprising vacuum-attracting tubes or a mechanical clamp structure 86 for holding the semiconductor chip 10 at its four corners from which none of the leads 12 extends, a vacuum-attracting mechanism for attracting a lower surface of the semiconductor chip 10, and an opening region. In operation, the semiconductor chip 10 is first held with the jig at the corners 18 and the lower surface thereof. Then, it is ascertained through the opening of the jig as to whether the bonding portions of the leads align with the bonding pads mounted on the substrate. Then, the bonding tool is made to pass through the opening of the jig to compress the leads onto the bonding pads with the semiconductor chip 10 being held by the clamp structure 86.
However, the above mentioned method cannot be applied to a semiconductor chip having leads extending from corners thereof, a small-sized semiconductor chip, and a semiconductor chip having leads extending therefrom in significantly high density.
In addition, since a sidewall of a semiconductor chip is not a vertically extending wall, it is almost impossible to directly hold a semiconductor chip at its corners by the above mentioned vacuum-attracting tube or mechanical clamp structure 86.
Furthermore, the jig as suggested in the Disclosure No. 4-277641 has to have a significantly complicated structure, and hence is not suitable for practical use. When a semiconductor chip is to be supported at its corners by a vacuum-attracting tube as that of the Disclosure No. 4-277641, such a vacuum-attracting tube has to extend horizontally. Since it is necessary for the vacuum-attracting tube to be connected to a tube which is in communication with a vacuum source, the jig has to be enlarged in size. It is difficult to have a space for such an enlarged jig. Similarly, when a semiconductor chip is to be supported at its corners by a mechanical clamp structure, a controller for driving the clamp has to be complicated, and hence it is unavoidable for the jig to be enlarged in size, resulting in it being not practical to incorporate the clamp structure into an outer lead bonding apparatus.